Self-immolating polymers (“SIPs”) are a relatively new class of stimuli-responsive polymers that can undergo controlled head-to-tail depolymerization with the cleavage of a triggering group. See, e.g., Peterson et al., Macromolecules 2012, 45:7317-7328; Wang et al., Angew. Chem. Int. Ed. 2008, 47:7804-7806; Avital-Shmilovici et al., Adv. Drug Deliv. Rev. 2011, doi:10.1016/j.addr.2011.09.012; Blencowe et al., Polym. Chem. 2011, 2:773-790. Different triggering groups have been developed that undergo enzyme, redox, nucleophile, acid/base, or photo-mediated reactions to release an electron rich species that can initiate a cascade of elimination reactions and ultimately deconstruct the macromolecule.
A desirable feature of SIP triggering groups is that they are stable to other forms of stimuli, ensuring that depolymerization occurs only under a specific set of conditions as necessary for its desired application. While thermally unstable polymers have been developed, they undergo degradation in a non-controlled manner via random chain thermolysis. See, e.g., Williams et al., Chem. Soc. Rev. 2007, 36:729-744. The thermal degradation of a poly(phthalaldehyde) above 150° C. is an example of a SIP undergoing thermally-induced depolymerization. See, e.g., Kostler et al., Polym. Chem. 2009, 47:1499; Coulembier et al., Macromolecules 2010, 43, 572-574; and Knoll et al., Adv. Mater. 2010, 22:3361-3365. However, random mid-chain cleavage occurs in the thermal degradation of these poly(phthalaldehyde)s, leading to multi-directional depolymerization as opposed to the site-controlled cleavage of a specific triggering group followed by head-to-tail depolymerization.
Heat has not been previously described as a practical stimulus for SIP triggers. A site-controlled cleavage of a specific heat-sensitive triggering group followed by head-to-tail depolymerization can ensure the polymer is fully degraded to monomers, allows for tunable triggering temperatures, and can maintain the option of side chain release (not yet achieved with poly(phthalaldehyde)). By developing a heat-sensitive thermal trigger, the utility of SIPs can be improved.